Despite efforts to slow the facility’s decline and correct structural problems, Kariba Dam is crumbling; a dam collapse at the site would prove disastrous for the entire region. By Jacques Leslie, February 2, 2016

The new year has not been kind to the hydroelectric-dam industry. On January 11th, the New York Times reported that Mosul Dam, the largest such structure in Iraq, urgently requires maintenance to prevent its collapse, a disaster that could drown as many as five hundred thousand people downstream and leave a million homeless. Four days earlier, the energy minister of Zambia declared that Kariba Dam, which straddles the border between his country and Zimbabwe, holding back the world’s largest reservoir, was in “dire” condition. An unprecedented drought threatens to shut down the dam’s power production, which supplies nearly half the nation’s electricity.

The news comes as more and more of the biggest hydroelectric-dam projects around the world are being cancelled or postponed. In 2014, researchers at Oxford University reviewed the financial performance of two hundred and forty-five dams and concluded that the “construction costs of large dams are too high to yield a positive return.” Other forms of energy generation—wind, solar, and miniature hydropower units that can be installed inside irrigation canals—are becoming competitive, and they cause far less social and environmental damage. And dams are particularly ill-suited to climate change, which simultaneously requires that they be larger (to accommodate the anticipated floods) and smaller (to be cost-effective during the anticipated droughts).

Mosul Dam’s predicament is partly a result of the ongoing war; many maintenance workers have not returned there since August of 2014, when ISIS fighters briefly took control. (Iraqi and Kurdish forces soon regained it.) But the main issue is that, like many such dams, the project shouldn’t exist in the first place. Opened in 1986, it was built on unstable gypsum bedrock, requiring grout to be constantly injected into the foundation to prevent the dam’s collapse. That work has ceased. In 2006, long before ISIS began making headlines, the U.S. Army Corps of Engineers called Mosul Dam “the most dangerous dam in the world.”

Kariba’s difficulties are more complicated. It has been nearly incapacitated by ongoing drought, which has lowered the reservoir’s volume to twelve per cent of its usual capacity. But if the reservoir is refilled, the dam faces the possibility of collapse. It was built in the late nineteen-fifties, and in the years since water flowing through the dam’s six floodgates has carved a three-hundred-foot-deep pit, or plunge pool, at its base. The plunge pool extends to within a hundred and thirty feet of the dam’s foundation; if it reaches the foundation, the dam will collapse. That seems hard to imagine now, with the reservoir at a record-low level. But the Zambezi River Basin, on which the dam sits, is the most susceptible of Africa’s thirteen basins to exceptional droughts and floods, and climate change is intensifying both.

Kariba’s collapse, like Mosul’s, would constitute an epochal event in the history of energy development—the dam industry’s Chernobyl. The ensuing torrent would be four times bigger than the Zambezi’s biggest recorded flood, in 1958, and would release enough water to knock over another major dam three hundred miles downstream, in Mozambique. At least three million people live in the flood’s path; most would die or lose their crops or possessions. About forty per cent of the electricity-generating capacity of twelve southern African nations would be eliminated.

The dam, four hundred and twenty feet tall and nearly two thousand feet wide, was built with financing from the World Bank to provide power for the copper mines of what was then Northern Rhodesia. The designers intended to make the dam impervious to a one-in-ten-thousand-year flood, but their calculations were based on only three decades of Zambezi flow data—a period too short to permit credible forecasting. This flaw became apparent in 1957, when the site, still under construction, was hit with a flood bigger than the designers’ worst-case projection. The planners hurriedly enlarged the spillway, but in 1958 the project was hit by another flood, twice as big as the previous one, so the spillway was expanded again. More recent projections, cited by the Intergovernmental Panel on Climate Change, indicate that the Zambezi River Basin will experience still drier and more prolonged droughts and even bigger floods in years to come.

Since the nineteen-nineties, Kariba’s operators have been allowed to open only three of the dam’s six floodgates at a time, for fear of enlarging the plunge pool. If the reservoir fills to a dangerous level, the operators face two options: allow water to flow over the top of the dam and tumble down its face, threatening the foundation, or, more likely, open more than three floodgates, causing the plunge pool to expand. To head off a catastrophe, the World Bank and other international lenders agreed in November of 2014 to provide a loan of nearly three hundred million dollars to repair it, stating that the project requires “immediate attention.”

But “immediate” means decades, not years. “It takes a long time to carry out the necessary due diligence and secure the financing for a complex project like this,” Munyaradzi Munodawafa, the chief executive of the Zambezi River Authority, which operates Kariba, told me in an e-mail. “We’re looking at a fifteen- to twenty-year process, in which we are five years under way.” To repair the dam, workers will enlarge the plunge pool downstream, to reduce erosion near the foundation. But it’s not certain to work, and major flooding could occur before the repairs are completed.

The World Bank and other international financiers like dams because they seem to offer large-scale solutions to energy and water shortages. Kariba is just one of more than two thousand large dams in Africa; Zimbabwe, one of the world’s poorest nations, has at least two hundred and fifty-four. But maintaining a dam is expensive—and much less popular than building one. Even in affluent countries such as the United States—whose dam infrastructure is in sufficient disrepair to have earned a “D” rating from the American Society of Civil Engineers, in 2013—maintenance is often neglected; it’s not likely to fare better in impoverished, corruption-ridden countries such as Zimbabwe or Iraq. Dams can’t be drained, and dismantling them can be as costly as building them. It’s the trap of Industrial Age technology: once mechanized systems supplant natural ones, they must be managed in perpetuity, or else they break down.